The field of the invention is clamps and the invention relates more particularly to clamps of the type useful for holding polymeric sheeting or other thin, flexible material.
Most modern greenhouses utilize two polymeric sheets which are adjacent at their edges and which are inflated therebetween to provide an insulative, and yet highly translucent, roofing. This permits the sunlight to pass through the roofing but still provides a highly insulative covering to exclude rain, insects and other elements which impair the optimum growing climate created within the greenhouse. Of course, it is important that the clamp or lock not form a line of stress for the sheeting which undergoes a certain amount of movement in the wind. It is also useful that the clamp provide shade for the polymeric sheeting at the line along which it is clamped so that the damaging ultraviolet rays will not weaken the sheeting at this critical point.
Extruded aluminum locks have found widespread use because of their ability to withstand outdoor weathering together with relatively low fabrication costs. The clamps typically have a lower member which is affixed to the frame of the greenhouse, and the sheeting is placed over the lower member. An upper member is then pressed into the lower member thereby holding the sheeting in place. One commonly used type of lock utilizes a separate metal clip to hold the upper member over the lower member. A preferred type of clamp has an upwardly-facing channel along one edge of the lower member and an upwardly extending contact bar along the other edge of the lower member. The upper member has a first downwardly facing contact bar which fits into the channel of the lower member and a second downwardly facing contact bar which abuts the outer surface of the contact bar on the lower clamp member. Although this clamp or lock is generally satisfactory, it has two significant drawbacks. First, the upper and lower members must both be made with a very close dimensional tolerance so that the upper member will stay securely on the lower member. Secondly, the upper member must be placed in the correct orientation in the lower member. If inserted incorrectly, it will fit into the lower member, but it will not hold the sheeting securely in place.